This device relates to an ultrasonic motor.
A structure of conventional ultrasonic motors is shown in FIG. 7. A support 8 is provided with a vibrator portion 82 to which a piezoelectric element 31 is adhered. A rotor 4 equipped with a bearing 9 is rotatably supported by a shaft portion 81 formed in the support 8 and which has balls 44. A coil spring 5 for bringing a bending plate portion 83 of the support 8 into pressure contact with the rotor 4 is provided around the shaft portion 91 of the support 8 in such a manner as to share the same vertical plane with the bearing 9 and moreover, to come into pressure contact with it. The other end of the coil spring 5 is supported by a spring pressing seat 6.
According to the conventional device, the coil spring 5 and a ball receiving sheet of the ball bearing are coaxially arranged so that they share the same vertical plane with each other. Therefore, this structure has a disadvantage for reduction of the thickness of ultrasonic motors. Since a means for restricting the ball position is not provided in the rotor or the ball receiving plate of the ball bearing, the balls are free to move to displaced positions, making the pressing force by the coil spring likely to be unstable.
Since the bearing lower ring is fixed to the rotor, friction occurs in the balls of the bearing due to the rotation of the rotor, thereby rotating a bearing upper ring. Accordingly, the pressing spring is twisted, the pressing force varies, and the bearing upper ring and the support pin are worn out, making the rotation of the rotor unstable. Furthermore, the position of the bearing lower ring supported turnably, with the shaft portion of the support pin of the rotor centered, is positioned lower than the projections of the vibration member in the cross section. Therefore, creaks occur between the inner ring and the support pin because of the inclination of the rotor center portion in the pressing direction. As a result, noise results and the force of the coil spring cannot be transmitted sufficiently to the contact portion between the projections of the vibration member and the rotor.
On the other hand, in a conventional travelling wave motor using a bearing for supporting the rotation of a rotor, the rotor is mounted rotatably on a central shaft via the bearing, In such a motor, the quantity of wriggling of the bearing is small and not set to a specific level.
In a travelling wave motor, two kinds of alternating voltages of different phases are applied to a piezoelectric element adhered to a vibration member, to generate a flexural travelling wave in a vibration member in accordance with an expansion motion of the piezoelectric element and drive a rotor, which is engaged under pressure with the vibration member. The rotor is rotated by a frictional force in a direction opposite to the direction in which the travelling wave advances. The contacting condition of the vibration member 303 and a friction member 302 bonded to the rotor 301 is as shown in FIG. 6, where A is the direction of rotor 301 and B is that of travelling wave. Since the rotor is driven by a frictional force, it is ideally desirable that all of the ridges of the travelling wave uniformly contact the sliding surface of the friction member. However, concerning the piezoelectric element, it is impossible to generate a consistently regular travelling wave due to the irregular shape of the electrode patterns and the irregular polarization condition thereof. Also, concerning the sliding surface of the friction member, it is impossible for the contacting condition thereof to be consistently uniform due to the lack of the manufacturing accuracy. Accordingly, in order to obtain a motor of a higher efficiency, it is necessary that the motor have a means for reducing the irregularities referred to above. In a motor using a bearing for supporting the rotation of a rotor, the wriggling portions of the bearing work to absorb such irregularities. Therefore, when the quantity of wriggling or play of a bearing is extremely small, such irregularity cannot be absorbed, so that the contacting condition becomes uneven. This causes a slip to occur, and the performance of the motor to be diminished.